Angular distributions have been measured for helium atoms scattering from isolated CO molecules chemisorbed on a Cu(001) surface as a function of incident beam energy between 9.4 and 100 meV and surface coverage from 1.3% ML to 9.3% ML. Up to five oscillations an clearly observed in the angular distributions. The parallel wave vector transfers of the peaks vary only slightly with incident energy and are independent of coverage up to 20% of a c(2 x 2) layer. New hard wall scattering calculations show that all of the distinct peaks observed can be explained by interference structures involving both Fraunhofer diffraction and illuminated face scattering from CO molecules with an approximate hard wall radius of 2.4 Angstrom with no evidence of the classical rainbows predicted in several recent theoretical studies.